Indirectly heated thermionic cathode



July 29, 1958 R. H-GEIGER INDIRECTLY HEATEDTHERMIONIC CATHODE Filed Feb.4, 1954 m. m4n m W0 0 m Unite States Patent INDIRECTLY HEATED THERMIGNICCATHODE Richard H. Geiger, Riveredge, N. 3., assignor to InternationalTelephone and Telegraph Corporation, Nutley, N. J., a corporation ofMaryland Application February 4, 1954, Serial No. 408,243

16 Claims. (Cl. 313-190) directly heated cathodes and indirectly heatedcathodes.

For the first type, which is essentially a resistance-heated cathode,tungsten or thoriated tungsten is commonly employed as the metallicelement. In order to insure an adequate emission of electrons, thesemetals must be brought to extremely high temperatures, and hence aredirectly heated. This offers the disadvantage that where tubes usingthese directly heated cathodes are employed as components of sensitiveamplifiers, undesirable hum and other unwanted noise may be introducedinto the amplifier because of the direct coupling of the heating elementin the circuit. Consequently, to avoid these undesired eifects, the useof indirectly heated oxide-coated cathodes has become common. Thesecathodes are indirectly heated usually by the radiation of heat from afilament maintained at a high temperature. Another method of heatingthese oxide-coated cathodes that has been utilized is the use ofbombardment-heated cathode arrangements. In these latter devices, afilament of an electron-emitting metal is heated to a high temperature,and this filament serves as an emitter of electrons which bombard theinternal or under surface of a coated cathode. The ensuing heatingeffect caused by the electron bombardment serves to make the cathodefunction as an indirectly heated one, with electrons being emitted fromthe external surface.

While the foregoing described cathodes are suitable for certainpurposes, they have associated with them certain inherent disadvantages.Thus, the filaments employed with indirectly heated cathodes of theconventional type are subject to burn out. Special refractory coatingsare required for these filaments. Where high current-carrying capacityis required, these filaments must be constructed of a large size andoperate at very high temperatures. Furthermore, they have a lowefficiency in that a relatively long length of cathode must be heated toobtain the desired emissivity from a narrower portion.

Often the refractory coating of the heater flakes off, resulting in ashort-circuit of the heater elements. If the cathode is of a complicatedstructure, the heat cannot be efficiently directed to the region whererequired.

It is therefore an object of the present invention to provide anindirectly heated cathode of considerably improved life compared withexisting indirectly heated cathodes.

It is a further object to provide an indirectly heated cathode suitablefor use in a gas discharge tube.

It is still a further object to provide a cathode structure capable ofbeing focused for the more eificient heating of specific areas of thecathode.

It is a feature of this invention that a cathode is thermionicallyheated by being subjected to a positive-ion bombardment resulting from acold cathode gas discharge.

Other objects and features of this invention may be seen from theaccompanying figures wherein:

Fig. 1 is a sectional view of an embodiment of a vacuum tube employingthis novel type cathode, and

Fig. 2 is a sectional view of another embodiment of a gas tube employinga similar type cathode.

Upon reference to Fig. 1, it is seen that the novel cathode structure 1is composed of an upper-cathodic portion 2, preferably made of nickel,whose upper or exterior surface 3 is coated with an oxide coating. Thismay be any of the coatings normally employed such as one resulting fromthe chemical reduction of a deposit of the mixed carbonates of barium,strontium and calcium to the corresponding oxides. The lower or internalface 4 of the cathode is coated with a secondary emission type coating.Any of various materials may be used for this purpose. Thus, I havefound that sputtered or evaporated metallic coatings of magnesium,barium, aluminum or cesium may be used. Also suitable for this purposeis an oxide coating such as one of aluminum oxide, magnesium oxide orbarium oxide. Another satisfactory coating is a mixture of cesium oxideand silver. This upper cathodic portion is usually copperbrazed to aglass-sealing alloy forming the lower portion 5 of the cathode. Asuitable alloy is one of nickeliron-cobalt containing a small percentageof manganese, commercially available as Kovar. The entire cathode isenclosed by means of a ceramic plug 6 at its lower end through which ananode structure 7 is extended. This structure may be of a desired shapeas indicated at 7a so as to aim, focus or direct a positive-ion beam ona specific portion of the secondary emission surface. For ease inevacuating the cathode enclosure and then adding a given ionizablemedium, it is preferable to use a hollow type anode whose end may beconveniently pinched off. This anode structure 7 is usually composed ofthe same metal, for example, Kovar, as the lower portion 5 of thecathode in order to simplify the forming Contained within this enoptimumperformance is a function of the desired tube.

voltages, the gas selected and the electrode geometry, and may bereadily determined by simple techniques known to those skilled in thisart. A suitable range of pressure is usually between approximately 1 and10 millimeters of mercury. Suitable non-electronegative gases for thispurpose are the rare gases, such as helium, neon, argon, krypton andxenon; and nitrogen, hydrogen and mercury vapor.

anode is connected to another external terminal 9. Heat shields 10 and11 may be provided to confine the electron emission of the cathode tothe principal anode 12 and thereby makefor more efiicient emission onthe part of the cathode. Cooling fins 13 are attached to the cathodestructure in order to radiate heat away from the ceramicto-metal sealwhich otherwise might be adversely affected. Upon sealing off thecathode enclosure filled with im ionizable medium, the remainder of thetube is highly evacuated and then sealed.

The vacuum tube is considered to operate as follows. Upon establishing adirect voltage or alternating voltage between electrode terminals 8 and9, the gas contained within the enclosed cathode structure is ionizedand com This gas is maintained at a pressure suitable for,

The outer wall of the cathode is connected toan external terminal 8which is usually grounded. Thev duction occurs. The secondary emissivesurface on the lower or internal face of the coated cathode wall servesto keep the cathode voltage drop relatively small. Inasmuch as thecathode is highly negative with respect to the anode 7, the positiveions of the contained gas are strongly attracted to the cathode face 4and, because of their relatively high molecular weight, serve to bombardit vigorously. This results in a heating of the cathode face to atemperature sufiicient for thermionic emission to occur from the upperemissive surface 3 of the cathode. The cathode 1 and the principal anode12 are contained within the evacuated envelope 14 composed of hard glassor quartz or similar material and are cooperatively disposed to operatein relation to one another in a manner customary with similar typevacuum tubes. Inasmuch as the principal feature of this invention isdirected to the provision of a novel type of indirectly heated cathode,it will be readily apparent to those skilled in the art that manymodifications may be made within the discharge tube itself withoutdeparting from the spirit of this invention. Thus, it will be obvious toprovide grid structures within the tube if triode, tetrode or pentodetype tubes are desired.

Similarly, many other modifications may be made in the materials ofconstruction of the tube without departing from the spirit of thisinvention. Thus, in place of the ceramic seal a Kovar-to-glass seal,using type 7052 glass, may be employed. In this instance, it would bepreferable to have the radiating cooling fins located directlysurrounding this coaxial type seal.

In Fig. 2, a gaseous discharge tube is illustrated, in contrast to thevacuum tube shown in Fig. 1, in which a related embodiment of thecathode structure is shown. Thus, in a manner similar to Fig. 1, thereis shown in Fig. 2 a cathode structure 15 containing an upper cathodeportion 16, composed preferably of nickel, whose upper or exteriorsurface 17 is coated with a mixed oxide coating of the oxides of barium,strontium and calcium, usually produced by chemical reduction of thecorresponding mixed carbonates. The inner face 18 is similarly coatedwith one of the secondary emission materialsas is used for coating face4 as described for Fig. 1. However, the cathode is not fully enclosed inthe embodiment shown in Fig. 2 because the gas contained within thecathode portion is of a similar type and at a similar pressure as thegas contained within the rest of the .gas tube. Again, thenon-electronegative type gases as described for Fig. 1 may be employed.This gas tube operates in a similar manner to that illustrated in Fig. 1in that in connecting the external cathode terminal 19 and the externalanode terminal 20 to a source of voltage, a discharge is set up betweenthe cathode 15 and the anode 21. Positive-ion bombardment of thesecondary emissive surface ensues, and thermionic emission occurs fromthe surface 17 upon this surface being brought to the requiredtemperature. Similar heating shields 22 and 23 are provided to increasethe cathode ei'ficiency by directing the emitted electrons to theprincipal anode 24 enclosed within the envelope 25. Because the gaswithin the principal section of the tube and within the cathodeenclosure is the same and at the identical pressure, the cathodeenclosure need not be sealed off from the principal portion of the tubebut rather includes a communicating passage 26 therebetween. Thiseliminates the need for metal-to-gl-ass or ceramic seals at this point,with the associated cooling fins. The anode 21 may be of a solid rodlikeconstruction since there is no need to separately evacuate and fill thecathode enclosure.

It will, of course, be realized that a certain amount of bombardment bypositive ions of the exterior emissive surface 17 will occur, but thismay be mitigated or otherwise controlled by changing the gas pressure orthe principal anode voltage, such as by adjusting variable resistor 27,or by adding additional electrodes and the like, these being techniquesWell known to those skilled 4 in the design of gas tubes. For certainpurposes, it may be desirable to have a gas discharge tube in which thegas pressure in the principal portion of the tube is different from thatwithin the cathode section. In such an event, the cathode may be made inthe form of a completely enclosed structure similar to that shown inFig. 1.

It will be seen from the foregoing figures that an indirectly heatedcathode has been provided which does not depend upon the presence of afilament heated to a highly elevated temperature. Thereby thepossibility of filament burn out has been eliminated. By using the coldcathode type of gas discharge as a source of positive ions, it ispossible to focus the bombardment upon a relatively small sector of thecathode structure. Such an arrangement becomes of importance whereintricate and involved cathode structures are required for certainspecialized applications. Obviously a gas type discharge can follow asinuous-shaped structure with considerable efficiency whereas afilamentary type heated structure cannot.

While I have described above the principles of my invention inconnection with specific apparatus, it is to be clearly understood thatthis description is made only by way of example and not as a limitationto the scope of my invention as set forth in the objects thereof and inthe accompanying claims.

I claim:

1. An evacuated electron discharge vessel, an anode within the vessel, acathode within the vessel containing emissive material cooperativelypositioned adjacent said anode and means for maintaining said emissivematerial at a temperature of thermionic emission, said means including.a cathode component which forms a wall part of a sealed-off dischargeenclosure containing a non-electronegative gas, said wall part and saidemissive material being in direct heat conducting relation, saidenclosure formed by said cathode component further including within thesaid enclosure an anode for initiating and maintaining a cold-cathodepositive-ion bombardment .of said wall portion to maintain the emissiveportion of said cathode at said temperature for thermionic emission anda lead to the anode disposed within the cathode housing and extendingthrough an opening in a second wall portion of said cathode housing, aseal of insulating material is disposed between said anode and saidsecond wall portion and heat-dissipating means are located adjacent tosaid seal to maintain it at a relatively low temperature.

2. An electron discharge vessel according to claim 1 wherein saidcathode housing is substantially cylindrical in shape.

3. A cathode for an electron discharge device .comprising a cathodehousing having an outer wall portion thereof coated at least in partwith an electron emissive material, an ionizable. medium contained insaid housing and an anode disposed within said housing to initiate andmaintain a cold-cathode positive-ion bombardment of said wall portion tothereby effect thermionic emission of said coated portion, said wallportion and said coated portion being in heat conducting relation, and alead to the anode disposed within the cathode housing and extendingthrough an opening in a second wall portion of said cathode housing, aseal of insulating material is disposed between said anode and saidsecond wall portion and heat-dissipating means are located adjacent tosaid seal to maintain it at a relatively low temperature.

4. A cathode for an electron discharge device comprising a cathodehousing having an outer wall portion thereof coated at least in partwith an electron emissive material, an ionizable medium contained insaid housing and an anode disposed within said housing to initiate andmaintain a cold-cathode positive-ion bombardment of said wall portion tothereby effect thermionic emission charge path including a main anodeand a thermionic cathode having a surface cooperating with said mainanode to provide said main discharge, means for heating s-aid thermioniccathode comprising means for providing an auxiliary discharge pathincluding an auxiliary anode, an auxiliary cathode surface and anon-electronegative gas in the path therebetween, said auxiliary cathodesurface being positioned in heat conducting relation with saidthermionic cathode surface and adapted to heat the same by saidauxiliary discharge, and means to mitigate bombardment of saidthermionic cathode by the positive ions of said non-electronegative gas.7

6. An electron discharge device comprising an envelope havingtherewithin means defining a main discharge path including a main anodeand a thermionic cathode having a surface cooperating with said mainanode to provide said main discharge, means for heating said thermioniccathode comprising means for providing an auxiliary discharge pathincluding an auxiliary anode, an auxiliary cathode surface and anionizable medium in the path therebetween, said auxiliary cathodesurface being positioned in heat conducting relation with saidthermionic cathode surface and adapted to heat the same by saidauxiliary discharge, and means to control the bombardment of saidthermionic cathode by the positive ions of said ionizable medium.

7. An electron discharge device comprising an envelope havingtherewithin means defining a main discharge path including a main anodeand a thermionic cathode having an electron emissive surface cooperatingwith said main anode to provide said main discharge, means for heatingsaid electron emissive surface to the thermionic emissive temperatureincluding a secondary emissive surface, a non-electronegative gas, andan auxiliary anode disposed adjacent said secondary emissive surface toinitiate and maintain a cold-cathode positiveion bombardment of saidsecondary emissive surface, said secondary emissive surface beingpositioned in heat conducting relationwith said electron emissivesurface and adapted to heat the same to said temperature by saidpositive-ion bombardment, and means to control the positive ionbombardment of said main cathode.

8. An electron discharge device comprising an envelope havingtherewithin a main anode, a cathode housing including on the exterior ofa wall portion thereof adjacent said main anode a coating of thermionicelectron emissive material to define together with said main tronemissive material to the thermionic emissive tempefa .ture including acoating of secondary emissive material on the interior of said wallportion in heat conducting relation with said electron emissivematerial, a non-electronegative gas, and an auxiliary anode disposedadjacent said secondary emissive material to initiate and maintain acold-cathode positive-ion bombardment of said secondary emissivematerial to heat said electron emissive material to said temperature,and means to control the positive ion bombardment of said main cathode.

9. A vacuum electron discharge device comprising a vacuum envelopehaving therewithin a main anode, a gas tight cathode housing includingon the exterior of a wall portion thereof adjacent said main anode acoating of thermionic electron emissive material to define together withsaid main anode a main discharge path and means for heating saidelectron emissive material to the thermionic emissive temperatureincluding a coating of seca 6 ondary emissive material on the interiorof said wall portion in heat conducting relation with said electronemissive material, a non-electronegative gas contained in said cathodehousing and an auxiliary anode disposed 'adjacentsaid secondary emissivematerial to initiate and maintain a cold-cathode positive-ionbombardment of said secondary emissive material to heat said electronemissive material to said temperature.

10. Adevice according to claim 9, wherein a lead to saidauxiliary anodeextends through an opening in a second wall portion of said cathodehousing, a seal ofinsulating material is disposed between said anode andsaid second wall portion and heat-dissipating means disposed adjacent tosaid seal to maintain it at a relatively low temperature.

11. A gaseous electron discharge device comprising a vacuum envelopehaving therewithin a non-electronegative gas, a main anode, a cathodehousing including on the exterior of a wall portion thereof adjacentsaid main anode a coating of thermionic electron emissive material todefine together with said main anode a main discharge path, means forheating said electron emissive material to the thermionic emissivetemperature including a coating of secondary emissive material on theinterior of said wall portion in heat conducting relation with saidelectron emissive material, communicating means between said cathodehousing and the principal space in said envelope in a second wallportion whereby said nonelectronegative gas can communicate with theinterior of said cathode housing and an auxiliary anode disposedadjacent said secondary emissive material to initiate and maintain acold-cathode positive-ion bombardment of said secondary emissivematerial to heat said electron emissive material to said temperature,and means to reduce the positive ion bombardment of said main cathode.

12. A device according to claim 11, wherein a lead to said auxiliaryanode extends through said communication means.

13. An indirectly heated thermionic cathode comprising a cathode housinghaving an outer wall portion thereof coated at least in part with anelectron emissive material, an ionizable medium within said housing, acoating of secondary emissive material on the interior of said wallportion in heat conducting relation with said electron emissive materialand an auxiliary anode disposed adjacent said secondary emissivematerial to initiate and maintain a cold-cathode positive-ionbombardment of said secondary emissive material to heat said electronemissive material to thermionic emissive temperatures.

14. A cathode according to claim 13, wherein said auxiliary anode isshaped and disposed in relation to said secondary emissive coating toeffectively focus a stream of positive ions thereon.

15. An indirectly heated thermionic cathode comprising a gas tightcathode housing having an outer wall portion thereof coated at least inpart with an electron emissive material, an ionizable medium disposedwithin said housing, a coating of secondary emissive material on theinterior of -said wall portion in heat conducting relation with saidelectron emissive material and an auxiliary anode disposed adjacent saidsecondary emissive material to initiate and maintain a cold-cathodepositive-ion bombardment of said secondary emissive material to heatsaid electron emissive material to thermionic emissive temperatures.

16. A cathode according to claim 15, wherein a lead to said auxiliaryanode extends through an opening in a second wall portion of saidcathode housing, a. seal of insulating material is disposed between saidanode and said second wall portion and heat-dissipating means disposedadjacent to said seal to maintain it at a relatively low temperature.

[References on following page) References Cited in the file of thispatent UNITED STATES PATENTS Stockle July 15, Ehert June 13, MuellerDec. 5, Mavrogenis Sept. 11, Smith June 21, Meyer et a]. Sept. 5, FoulkeJuly 10,

Smith Nov. 15,

8 2,201,817 Smith May 21, 1940 2,201,818 Smith May 21, 1940 2,201,819Smith May 21, 1940 V I FOREIGN PATENTS 384,099 Great Britain Dec. 1,1932 OTHER REFERENCES Serial No. 389,323, Herriger (A. I. 0.), published

